The present invention relates generally to electro-optic transmitters, receivers, and transceivers, and more particularly, to an electromagnetic interference reduction method and apparatus for use with such transmitters, receivers, and transceivers.
With the growth of computer networks (e.g., the Internet and the World Wide Web), the demand for network devices that form these networks is also rapidly increasing. A measure of performance of these network devices is the rate or speed at which the devices transfer data. As the information that needs to be transferred across the network becomes more voluminous and complex (e.g., the distribution of audio files, video files, and software programs over the network), networking companies are constantly challenged to design and manufacture networking products (e.g., switches and routers) that push the envelope in terms of the speed at which these products transfer data.
One way to enhance the speed of distributing data across a network is to utilize light to carry information and to use fiber optics as a transport mechanism. One advantage of using fiber optics is that a greater volume of data can be transferred at higher speeds via fiber optic cables as compared to electrical wires. However, signals are typically generated by a digital system (e.g., a web browser program executing on a computer) in an electrical format. Consequently, one important component in these networking devices is an optical module for converting electrical signals into optical signals and vice versa.
Examples of optical modules are optical receivers, optical transmitters, and optical transceivers. An optical receiver is a circuit that receives light signals from an optical medium (e.g., an optical fiber cable) and converts these light signals into corresponding electrical signals. An optical transmitter is a circuit that receives electrical signals and converts these electrical signals into corresponding light signals suitable for transmission across an optical medium, such as fiber optic cables. When a circuit includes both the functionality of an optical receiver and an optical transmitter, the circuit is commonly referred to as an optical transceiver.
Another important design consideration for manufacturers of electrical products is to ensure that the device complies with governmental regulations concerning electrical noise. One such regulation is promulgated by the Federal Communication Commission (FCC), Part 15, Class A that specifies the amount of allowed electromagnetic interference (EMI) that can escape from an electronic device. Devices that are distributed in other jurisdictions often need to comply with that jurisdiction""s regulations concerning EMI emission. For example, products distributed in Europe typically need to comply with the International Special Committee on Radio Interference (CISPR) Publication 22 that specifies the amount of EMI emission allowed for electronic products.
In current designs, a chassis for housing the electrical-optical components of a networking device is made of a conductive material (e.g., a metal). The metal effectively prevents any EMI generated by the components of the networking device from escaping from the inside of the chassis (i.e., the metal effectively confines EMI to inside the chassis). However, the chassis has a plurality of openings formed therein for providing access to the inside of the chassis. Examples of these openings include an opening for receiving the power cord, openings for ventilation, etc. An important type of such openings are openings in the chassis for receiving the optical modules (e.g., optical transmitter, optical receiver, and optical transceiver) described previously that provide an interface between an optical medium (e.g., a fiber optic cable) and a laser (e.g., a Vertical Cavity Surface Emitting Laser (VCSEL)) or a photodiode (e.g., a PIN diode).
The optical module includes a surface for defining a slot for allowing light to pass there through so that the light signals described above can be communicated with the components of the network device. The slot has disposed therein an optical element that is typically made of a non-conductive material, such as plastic. The optical element provides an optical interface between the laser and the fiber optic cable that typically re-directs, focuses and launches the light into the fiber optic cable. The optical element does not prevent EMI from escaping out of the chassis. However, when the devices operate at data rates in the megabit range, the wavelength of the EMI is sufficiently large with respect to the physical dimensions of the slot so as to prevent significant EMI from escaping.
Unfortunately, as the data rates of the fiber optic-based communications increase into the gigabit range, the wavelength of the EMI decreases relative to the physical dimensions of the slot. Consequently, EMI can now escape more efficiently through the slot with current dimensions at these higher frequencies. As described hereinafter, there is trend in the industry to increase the dimensions of the slot, thereby further aggravating the escape of EMI.
There is a trend in the industry to increase the amount of data that can be transferred by using multiple channels instead of a single channel in a transmitter, receiver, or transceiver. An exemplary optical module can include an array of lasers (e.g., an array of VCSELs) for use with a multi-channel fiber optic cable, where the number of channels corresponds to the number of lasers. For example, a typical optical transmitter includes a laser array (e.g., a VCSEL array) and a laser driver circuit for receiving input signals and translating the input signals into voltage levels suitable to drive the laser array.
By increasing the number of channels in the fiber optic cable and the number of lasers in the optical module, more data can be transmitted by the optical transmitter. For example, by having two parallel channels, twice as much data can be transmitted through the optical module. Unfortunately, this trend tends to increase the dimensions of the slot that must now accommodate an array of optical elements instead of a single optical element. Each optical element in the array of optical elements optically interfaces a particular channel in the multi-channel fiber optic cable to a corresponding laser in the laser array.
In summary, the increased data speeds results in EMI having a smaller wavelength that can better escape a given dimension for the slot in the optical modules. Also, the advent of multi-channel optic fiber, arrays of lasers, arrays of photodiodes and arrays of corresponding optical elements has the potential to increase the dimensions of the slot to accommodate the array of optical elements. The decreasing wavelength of the EMI together with the increasing dimensions of the slot magnify the likelihood that significant EMI would escape from the inside of the chassis through the slot. The increase in EMI escape can cause a device to fail the stringent requirements of the governmental regulations concerning EMI emission.
Currently, there is no mechanism for reducing EMI emission at high data rates for a slot that handles a single optical element and for addressing the problem of increased slot dimensions for accommodating a set of optical elements. Consequently, there is a need for a cost-effective solution to reduce EMI emission that can be incorporated efficiently into the optical module.
Based on the foregoing, there remains a need for an electromagnetic interference shielding method and apparatus that overcomes the disadvantages set forth previously.
According to one aspect of the present invention, a method and system for reducing electromagnetic interference (EMI) that escapes a chassis for housing electro-optic components are provided. In one embodiment, the chassis has a plurality of openings for receiving optical modules. Each optical module has a slot with a first dimension and a second dimension through which EMI can escape. An optical array is disposed in the slot. The optical array has a plurality of optical elements and a first dimension. At least one strip of conducting material is positioned between two optical elements at predetermined intervals as measured along the first dimension. The strip has a first end and a second end and is configured to extend substantially the second dimension. At least one of the first end and the second end are coupled to a ground potential. The strips effectively reduce the amount of EMI that escapes from the slot.
In an alternative embodiment, the chassis has a plurality of openings for receiving optical modules. Each optical module has a slot with a first dimension and a second dimension through which EMI can escape. An optical array is disposed in the slot. The optical array has a plurality of optical elements and a first dimension. At least one strip of conducting material is positioned across the optical element. The strip is configured to extend substantially the first dimension, is coupled to a ground potential, and effectively reduces the EMI that escapes from the slot.